The most important components of conventional EUV optical systems are Bragg mirrors. In the case of the material combinations used nowadays for the abovementioned mirrors, in particular molybdenum-silicon multilayers, a broadband nature and a high reflectivity are largely ruled out for the following reason: each individual layer reflects only little light, such that high reflectivities require a multiplicity (often 60 or even more) of layers whose reflections are constructively superimposed. These high numbers of layers have the effect that the Bragg mirrors become spectrally narrowband. For EUV scanners in optical lithography, owing to the high throughput sought, it is important, however, to employ highly reflective mirrors in order that as much light as possible arrives at the wafer.
The literature discloses measurements in which various spectra were used to examine resolution limits of an EUV microscope (see e.g. Wachulak et al. 2011, Optics Express 19, pages 9541-9549).